U.S. patent number 8,663,380 [Application Number 11/941,640] was granted by the patent office on 2014-03-04 for gas phase production of coated titania.
This patent grant is currently assigned to Cristal USA Inc.. The grantee listed for this patent is M. Kamal Akhtar, Alexandra Teleki Harsanyi, Martin Christian Heine, Sotiris Emmanuel Pratsinis, Reto Strobel. Invention is credited to M. Kamal Akhtar, Alexandra Teleki Harsanyi, Martin Christian Heine, Sotiris Emmanuel Pratsinis, Reto Strobel.
United States Patent |
8,663,380 |
Akhtar , et al. |
March 4, 2014 |
Gas phase production of coated titania
Abstract
A flame spray pyrolysis process for the preparation of ultrafine
titania particles coated with a smooth, homogeneous coating of one
or more metal oxides is provided. The metal oxide coating is
achieved by contacting the titania particles with a metal oxide
precursor downstream of the titania formation zone, after the
titania particles have formed. The process provides titania
particles with a high rutile content and a smooth and homogeneous
coating of a metal oxide.
Inventors: |
Akhtar; M. Kamal (Ellicott
City, MD), Pratsinis; Sotiris Emmanuel (Zurich,
CH), Heine; Martin Christian (Seuzach, CH),
Harsanyi; Alexandra Teleki (Zurich, CH), Strobel;
Reto (Zurich, CH) |
Applicant: |
Name |
City |
State |
Country |
Type |
Akhtar; M. Kamal
Pratsinis; Sotiris Emmanuel
Heine; Martin Christian
Harsanyi; Alexandra Teleki
Strobel; Reto |
Ellicott City
Zurich
Seuzach
Zurich
Zurich |
MD
N/A
N/A
N/A
N/A |
US
CH
CH
CH
CH |
|
|
Assignee: |
Cristal USA Inc. (Hunt Valley,
MD)
|
Family
ID: |
40639068 |
Appl.
No.: |
11/941,640 |
Filed: |
November 16, 2007 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20090126604 A1 |
May 21, 2009 |
|
Current U.S.
Class: |
106/436; 106/442;
427/453 |
Current CPC
Class: |
C04B
41/009 (20130101); C04B 41/81 (20130101); C09C
1/3661 (20130101); C04B 41/4584 (20130101); C01G
23/07 (20130101); C23C 16/401 (20130101); C23C
16/4417 (20130101); B82Y 30/00 (20130101); C04B
41/4584 (20130101); C04B 41/4531 (20130101); C04B
41/5035 (20130101); C04B 41/4584 (20130101); C04B
41/4531 (20130101); C04B 41/5027 (20130101); C04B
41/009 (20130101); C04B 35/46 (20130101); C01P
2004/04 (20130101); C01P 2006/12 (20130101); C01P
2002/54 (20130101); C01P 2004/64 (20130101); C01P
2004/84 (20130101) |
Current International
Class: |
C09C
1/36 (20060101); C23C 4/10 (20060101) |
Field of
Search: |
;106/436,438,441-442
;427/453 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1042408 |
|
May 2009 |
|
EP |
|
WO 96/36441 |
|
Nov 1996 |
|
WO |
|
WO 2003/070640 |
|
Aug 2003 |
|
WO |
|
WO 2006/116887 |
|
Nov 2006 |
|
WO |
|
WO2007/028267 |
|
Mar 2007 |
|
WO |
|
Other References
Akhtar, M. K., S. E. Pratsinis, and S. V. R. Mastrangelo, "Dopants
in vapor-phase synthesis of titania powders," J. Am. Ceram. Soc.
75, 3408 (1992). cited by applicant .
Akhtar, M. K., S. E. Pratsinis, and S. V. R. Mastrangelo,
"Vapor-phase synthesis of Al-doped titania powders," J. Mater. Res.
9, 1241 (1994). cited by applicant .
Allen, N. S., H. Khatami, and F. Thompson, "Influence of
titanium-dioxide pigments on the thermal and photochemical
oxidation of low-density polyethylene film," Eur. Polym. J. 28,
817, 1991. cited by applicant .
Allen, N. S., M. Edge, G. Sandoval, J. Verran, J. Stratton, and J.
Maltby, "Photocatalytmonthic coatings for environmental
applications," Photochem. Photobiol. 81, 279 (2005). cited by
applicant .
Braun, J. H., "Titanium dioxide--A review," J. Coat. Technol. 69,
59 (1997). cited by applicant .
Carotenuto, G., Y. S. Her, and E. Matijevic, "Preparation and
characterization of nanocomposite thin films for optical devices,"
Ind. Eng. Chem. Res. 35, 2929 (1996). cited by applicant .
Caseri, W. R., "Nanocomposites of polymers and inorganic particles:
preparation, structure and properties," Mater. Sci. Technol. 22,
807 (2006). cited by applicant .
Cheary, R. W., and A. A. Coelho, "Axial divergence in a
conventional X-ray powder diffractometer. I. Theoretical
foundations," J. Appl. Crystallogr. 31, 851 (1998). cited by
applicant .
Christensen, P. A., A. Dilks, T. A. Egerton, and J. Temperley,
"Infrared spectroscopic evaluation of the photodegradation of
paint--Part II: The effect of UV intensity & wavelength on the
degradation of acrylic films pigmented with titanium dioxide," J.
Mater. Sci. 35, 5353 (2000). cited by applicant .
Egerton, T. A., "The modification of fine powders by inorganic
coatings," KONA 16, 46 (1998). cited by applicant .
El-Toni, A. M., S. Yin, and T. Sato, "Control of silica shell
thickness and microporosity of titania-silica core-shell type
nanoparticles to depress the photocatalytic activity of titania,"
J. Colloid Interface Sci. 300, 123 (2006). cited by applicant .
Hung, C. H., and J. L. Katz, "Formation of mixed-oxide powders in
flames 1. TiO.sub.2-5iO.sub.2," J. Mater. Res. 7, 1861 (1992).
cited by applicant .
Kodas, T. T., Q. H. Powell, and B. Anderson, Coating of TiO.sub.2
pigment by gas phaseand surface reactions, International patent, WO
96/36441 (1996). cited by applicant .
Kyprianidou-Leodidou, T., P. Margraf, W. Caseri, U. W. Suter, and
P. Walther, "Polymer sheets with a thin nanocomposite layer acting
as a UV filter," Polym. Adv. Technol. 8, 505 (1997). cited by
applicant .
Lee, S. K., K. W. Chung, and S. G. Kim, "Preparation of various
composite TiO.sub.2/SiO.sub.2 ultrafine particles by vapor-phase
hydrolysis," Aerosol Sci. Technol. 36, 763 (2002). cited by
applicant .
Madler, L., H. K. Kammler, R. Mueller, and S. E. Pratsinis,
"Controlled synthesis of nanostructured particles by flame spray
pyrolysis," J. Aerosol Sci. 33, 369 (2002). cited by applicant
.
Madler, L., W. J. Stark, and S. E. Pratsinis, "Simultaneous
deposition of Au nanoparticles during flame synthesis of TiO.sub.2
and SiO.sub.2," J. Mater. Res. 18, 115 (2003). cited by applicant
.
Mezey, E. J., "Pigments and Reinforcing Agents" in Vapor
Deposition, eds. C. F. Powell, J. H. oxley and J. M. Blocher Jr.,
John Wiley & sons, New York (1966). cited by applicant .
Nussbaumer, R. J., W. R. Caseri, P. Smith, and T. Tervoort,
"Polymer-TiO.sub.2 nanocomposites: A route towards visually
transparent broadband UV filters and high refractive index
materials," Macromol. Mater. Eng. 288, 44 (2003). cited by
applicant .
Powell, Q. H., G. P. Fotou, T. T. Kodas, B. M. Anderson, and Y. X.
Guo, "Gas-phase coatin of TiO.sub.2 with SiO.sub.2 in a continuous
flow hot-wall aerosol reactor," J. Mater. Res. 12, 552 (1997).
cited by applicant .
Schulz, H., L. Madler, R. Strobel, R. Jossen, S. E. Pratsinis, and
T. Johannessen, "Independent control of metal cluster and ceramic
particle characteristics during one-step synthesis of
Pt/TiO.sub.2," J. Mater. Res. 20, 2568 (2005). cited by applicant
.
Song, K. C., and S. E. Pratsinis, "Synthesis of bimodally porous
titania powders by hydrolysis of titanium tetraisopropoxide," J.
Mater. Res. 15, 2322 (2000). cited by applicant .
Teleki, A., S. E. Pratsinis, K. Kalyanasundaram, and P. I. Gouma,
"Sensing of organic vapors by flame-made TiO.sub.2 nanoparticles,"
Sens. Actuators, B, Chem 119, 683 (2006). cited by applicant .
Teleki, A., S. E. Pratsinis, K. Wegner, R. Jossen, and F. Krumeich,
"Flame-coating of titania particles with silica," J. Mater. Res.
20, 1336 (2005). cited by applicant .
Vemury, S., and S. E. Pratsinis, "Dopants in flame synthesis of
titania," J. Am. Ceram. Soc. 78, 2984 (1995). cited by applicant
.
Wegner, K., and S. E. Pratsinis, "Nozzle-quenching process for
controlled flame synthesis of titania nanoparticles," AIChE J. 49,
1667 (2003). cited by applicant .
Fotou, George P. et al., "Coating Titania Aerosol Particles with
ZrO.sub.2, Al.sub.2O.sub.3/ZrO.sub.2, and SiO.sub.2/ZrO.sub.2 in a
Gas-Phase Process," Aerosol Science and Technology 33:557-571,
2000, American Associate for Aerosol Research Published by Taylor
and Francis. cited by applicant .
Swihart, Mark T., "Vapor-phase synthesis of nanoparticles " Current
Opinion in Colloid and Interface Science 8, 2003, pp. 127-133.
cited by applicant .
PCT/US08/80799, International Search Report and Written Opinion,
Dec. 24, 2008. cited by applicant .
Ortega et al., "Control of Particle Morphology During
Multicomponent Metal Oxide Powder Generation by Spray Pyrolysis" J.
Aerosol Sci., vol. 23, Suppl. 1, (1992) pp. S253-S256. cited by
applicant .
Strobel et al., "Flame Aerosol Synthesis of Smart Nanostructured
Materials" J. Mater. Chem., vol. 17, (2007) pp. 4743-4756. cited by
applicant .
Song, Shin Ae, and Park, Seung Bin, "Synthesis of Silica-Coated
Ceria Particles for STI-CMP in a Single Step by Flame Spray
Pyrolysis with an Emulsion" Journal of the Electrochemical Society,
(2011) vol. 158, Issue 8, pp. K170-K174. (abstract). cited by
applicant.
|
Primary Examiner: Parvini; Pegah
Attorney, Agent or Firm: Dunlap Codding, P.C.
Claims
What is claimed is:
1. A method for the preparation of coated ultrafine titanium
dioxide particles, the method comprising: (a) reacting a TiO.sub.2
precursor with oxygen in a flame spray pyrolysis reactor to form
ultrafine titanium dioxide particles in a TiO.sub.2 formation zone
of the flame spray pyrolysis reactor, the ultrafine titanium
dioxide particles having an average particle size in a range of 1
nm to 100 nm; and (b) contacting the ultrafine titanium dioxide
particles with a metal oxide precursor in a non-reactive carrier
gas downstream of the TiO.sub.2 formation zone at a point in the
flame spray pyrolysis reactor where the temperature is less than
about 900.degree. C., to form coated ultrafine titanium dioxide
particles having a smooth, homogeneous metal oxide coating.
2. The method of claim 1 further comprising isolating the coated
ultrafine titanium dioxide particles.
3. The method of claim 1 wherein separate amorphous metal oxide
particles are not formed from the metal oxide precursor.
4. The process of claim 1, wherein the TiO.sub.2 precursor is
selected from the group consisting of titanium halides and titanium
alkoxides.
5. The process of claim 4, wherein the TiO.sub.2 precursor is
selected from the group consisting of TiCl.sub.4, TiCl.sub.3,
titanium tetraisopropoxide, titanium tetraethoxide, and titanium
tetrabutoxide.
6. The process of claim 1, wherein the coated ultrafine titanium
dioxide particles are at least 95% by weight in the rutile
form.
7. The process of claim 1, wherein the ultrafine titanium dioxide
particles comprise aluminum oxide.
8. The process of claim 1, wherein the average particle size of the
ultrafine titanium dioxide particles is between about 15 nm to
about 80 nm.
9. The process of claim 1, wherein the average particle size of the
ultrafine titanium dioxide particles is between about 15 nm to
about 60 nm.
10. The process of claim 1, wherein the specific surface area of
the ultrafine titanium dioxide particles is between about 15
m.sup.2/g to about 100 m.sup.2/g.
11. The process of claim 1, wherein the metal oxide coating
comprises a metal oxide selected from the group consisting of
SiO.sub.2, Al.sub.2O.sub.3, B.sub.2O.sub.3, ZrO.sub.2, GeO.sub.2,
WO.sub.3, Nb.sub.2O.sub.5, MgO, ZnO, and SnO.sub.2.
12. The process of claim 11, wherein the metal oxide is
SiO.sub.2.
13. The process of claim 11, wherein the metal oxide is
Al.sub.2O.sub.3.
14. The process of claim 1, wherein the ultrafine titanium dioxide
particles are contacted with the metal oxide precursor at a point
downstream of the TiO.sub.2 formation zone where at least 90% of
the TiO.sub.2 precursor has reacted with oxygen to the form
ultrafine titanium dioxide particles.
15. The process of claim 1, wherein the ultrafine titanium dioxide
particles are contacted with the metal oxide precursor at a point
downstream of the TiO.sub.2 formation zone where at least 95% of
the TiO.sub.2 precursor has reacted with oxygen to form the
ultrafine titanium dioxide particles.
16. The process of claim 1, wherein the metal oxide precursor is
selected from the group consisting of silicon halides,
hexaalkyldisiloxanes, tetraalkylorthosilicates and silicon
containing salts.
17. The process of claim 16, wherein the metal oxide precursor is
hexamethyldisiloxane.
18. The process of claim 1, wherein the metal oxide coating has a
thickness in a range of about 1 nm to about 10 nm.
19. The process of claim 1 wherein the metal oxide coating has a
thickness in a range of about 2 nm to about 4 nm.
20. A method for the preparation of SiO.sub.2-coated ultrafine
titanium dioxide particles, the method comprising: (a) reacting a
TiO.sub.2 precursor with oxygen in a flame spray pyrolysis reactor
to form ultrafine titanium dioxide particles in a TiO.sub.2
formation zone of the flame spray pyrolysis reactor, the ultrafine
titanium dioxide particles having an average particle size in a
range of 1 nm to 100 nm; and (b) contacting the ultrafine titanium
dioxide particles with a SiO.sub.2 precursor in a non-reactive
carrier gas downstream of the TiO.sub.2 formation zone at a point
in the flame spray pyrolysis reactor where the temperature is less
than about 900.degree. C., to form SiO.sub.2-coated ultrafine
titanium dioxide particles wherein the SiO.sub.2 coating on the
ultrafine titanium dioxide particles is smooth and homogeneous.
21. The method of claim 20 wherein separate amorphous SiO.sub.2
particles are not formed from the SiO.sub.2 precursor.
22. The process of claim 20, wherein the particle size of the
ultrafine titanium dioxide particles is between about 15 nm to
about 80 nm.
23. The process of claim 20, wherein the particle size of the
ultrafine titanium dioxide particles is between about 15 nm to
about 60 nm.
24. The process of claim 20, wherein the ultrafine titanium dioxide
particles are contacted with the SiO.sub.2 precursor at a point
downstream of the TiO.sub.2 formation zone where at least 95% of
the TiO.sub.2 precursor has reacted with oxygen to the form
ultrafine titanium dioxide particles.
25. The process of claim 20, wherein the SiO.sub.2 precursor is
selected from the group consisting of silicon halides,
hexaalkyldisiloxanes, tetraalkylorthosilicates and silicon
containing salts.
26. The process of claim 25, wherein the SiO.sub.2 precursor is
hexamethyldisiloxane.
27. The process of claim 20 wherein the SiO.sub.2 coating has a
thickness in a range of about 2 nm to about 4 nm.
Description
FIELD OF THE INVENTION
This invention relates to a high temperature gas-phase process for
the production of ultrafine titania particles coated with a
homogeneous metal oxide layer.
BACKGROUND OF THE INVENTION
Titania particles possess an attractive combination of optical
properties such as absorption of ultraviolet light and a very high
refractive index. Polymer composites comprising inorganic
nanoparticles are attractive for a range of optical applications
(Carotenuto et al., 1996). Undesired light scattering (Beecroft and
Ober, 1997) is significantly reduced in nanocomposites compared to
composites containing larger particles (>50 nm;
Kyprianidou-Leodidou et al., 1997) if the refractive indices of
polymer and particles differ and if the primary particles are
randomly distributed in the polymer matrix. These nanocomposites
will often appear transparent (Caseri, 2006). Rutile
TiO.sub.2-based nanocomposites can be used as UV filters, coatings
for UV-sensitive materials and lenses as the particles absorb UV
light, are transparent at the visible wavelengths and possess a
high refractive index (Nussbaumer et al., 2003). The anatase phase
is less suited for these applications as its absorption edge is
located at lower wavelengths (Christensen et al., 2000) and
generally has a higher photocatalytic activity which can lead to
degradation of the polymer matrix (Allen et al., 1992).
A significant amount of the world's annual production of titania is
produced by the gas phase oxidation of titanium tetrachloride using
a pyrolysis process. In flame synthesis of titania, an industrial
process for the production of pigmentary titania (Braun, 1997),
typically the anatase phase is formed under oxygen-rich conditions
at atmospheric pressure (Wegner and Pratsinis, 2003). Rutile can be
synthesized by thermal treatment of anatase titania (Song and
Pratsinis, 2000), however this also leads to grain growth and
agglomeration. The larger particles or agglomerates significantly
scatter light (Beecroft and Ober, 1997) resulting in opaque
composites (Nussbaumer et al., 2003; Caseri, 2006). Rutile
formation during synthesis of titania nanoparticles can be promoted
by co-oxidation with aluminum precursors (Mezey et al., 1966) as
has been shown in hot-wall (Akhtar et al. 1994) and diffusion flame
(Vemury and Pratsinis 1995) aerosol reactors.
The titania particle surface can be passivated by coatings, in
order to prevent the phototcatalytic decomposition of the polymer
matrix (El-Toni et al., 2006). Coatings can reduce the generation
of free radicals by physically inhibiting oxygen diffusion,
preventing the release of free radicals and providing hole-electron
or hydroxyl-radical recombination sites (Allen et al., 2005).
Furthermore, coatings can also improve wetting and dispersion
properties of the particles in an organic matrix (Egerton, 1998;
Allen et al., 2005). Coatings are typically applied to pigmentary
titania in a post-synthesis, wet-phase treatment by precipitation
of nano-sized hydrous oxides of Al, Zr, Sn or Si onto the titania
surface (Iler, 1959). Silica coating of titania is particularly
attractive because this coating yields maximum durability of the
coated material. However, this is also accompanied by loss of
opacity as a result of agglomeration during wet-phase treatment.
Wet dispersion of the starting powder, filtration, washing and
drying add to production time and cost. Furthermore, the control of
the coating morphology is difficult in the wet precipitation
process. Rough and porous coatings are often obtained where
complete and homogeneous coatings are desired for optimum
durability and a maximum reduction of photoactivity of the
titania.
In-situ gas-phase processes have been investigated as alternative
coating routes either in aerosol flow (Piccolo et al., 1977) or
flame reactors (Hung and Katz, 1992). In flame reactors SiO.sub.2
coated TiO.sub.2 can be formed by co-oxidation of silicon and
titanium precursors (Hung and Katz, 1992; Teleki et al., 2005). The
product powder morphology is a result of simultaneous growth of the
two oxides in the flame and can be controlled by precursor
concentration and flame temperature (Hung and Katz, 1992). In a
diffusion flame rapid cooling of particle growth by nozzle
quenching (Wegner and Pratsinis, 2003) facilitated the formation of
smooth silica coatings while in the unquenched flame mainly
particles segregated in silica and titania were formed (Teleki et
al., 2005). In aerosol flow reactors coating precursors can be
added downstream a TiO.sub.2 particle formation zone to produce
oxide coatings on the titania nanoparticles (Kodas et al., 1996;
Powell et al., 1997). The key process parameters controlling
coating morphology are temperature and coating precursor
concentration (Powell et al., 1997) as well as the mixing mode of
titania particles and coating precursor (Lee et al., 2002).
U.S. Pat. No. 5,562,764 to Gonzalez describes a process for
producing substantially anatase-free TiO.sub.2 by addition of a
silicon halide to the reaction product of TiCl.sub.4 and an oxygen
containing gas in a plug flow reactor. The silicon halide is added
downstream of where the TiCl.sub.4 and oxygen gas are reacted. The
patent describes a process to produce pigmentary grade TiO.sub.2,
and the SiCl.sub.4 is added to the process at a temperature of
about 1200.degree. C. to about 1600.degree. C., and a pressure of
5-100 psig.
International Application Publication No. WO 96/36441 to Kemira
Pigments, Inc. describes a process for making pigment grade
TiO.sub.2 coated with a metal oxide in a tubular flow reactor. The
metal oxide precursor is introduced downstream of the TiO.sub.2
formation zone. The publication discloses that the temperature for
treating TiO.sub.2 with a silica precursor must be sufficiently
high to ensure that the precursor forms SiO.sub.2. The publication
discloses that for coating TiO.sub.2 with SiO.sub.2 using
SiCl.sub.4, the temperature must be greater than 1300.degree. C.
The particles produced by the process are pigment grade.
U.S. Pat. No. 6,562,314 to Akhtar et al., describes a process for
the production of substantially anatase-free TiO.sub.2 by
introducing a silicon compound into the TiCl.sub.4 stream to form
an admixture before the reaction with oxygen. The process is
conducted under pressure and the titania is not coated with
silica.
U.S. Pat. Nos. 6,852,306 and 7,029,648 to Subramanian et al.,
describe a process to produce TiO.sub.2 pigment particles coated
with silica in a tubular flow reactor. The TiCl.sub.4 is introduced
downstream of the TiO.sub.2 formation zone at a temperature of no
greater than 1200.degree. C. The coating produced by this about 1
to 4 nm thick and is a mixture of amorphous aluminum oxide and
amorphous silicon dioxide. Only silicon halides are used as the
metal oxide precursor.
U.S. Pat. No. 7,083,769 to Moerters et al., describes
silicon-titanium mixed oxide powders prepared by a flame hydrolysis
process. The process described comprises introducing streams of
TiCl.sub.4 and a silica precursor into the burner at the same time.
The mixed oxide produced is disclosed to be an intimate mixture of
titanium dioxide and silicon dioxide on an atomic level with the
formation of Si--O--Ti bonds. The surface of the particles is
disclosed to be enriched with silicon.
U.S. Pat. No. 6,328,944 to Mangold et al., describes doped metal
oxides or non-metal oxides prepared by a process which comprises
feeding aerosols into the flame of a pyrogenic reactor. SiCl.sub.4
is fed into a combustion chamber via one feed tube and an aerosol
which comprises a second metal oxide dopant is fed to the
combustion chamber separately through another tube. The SiCl.sub.4
and the dopant aerosol are mixed together prior to reaching the
combustion chamber.
Although these gas-phase coating techniques offer promise in
obtaining metal oxide coated titania particles with desired
characteristics, the production of titania nanoparticles with
smooth homogeneous metal oxide coatings remains a challenge.
Therefore, there remains a need for a pyrolysis process that
produces high rutile content titanium dioxide nanoparticles that
are coated with a smooth and homogeneous coating of a second metal
oxide layer.
SUMMARY OF THE INVENTION
Provided is a process for the preparation of ultrafine titanium
dioxide particles comprising a smooth homogeneous coating of one or
more metal oxides on the surface of the titanium oxide particles
comprising: (a) reacting a TiO.sub.2 precursor with oxygen in a gas
phase oxidation reactor to produce ultrafine TiO.sub.2 particles;
(b) contacting the ultrafine TiO.sub.2 particles with a metal oxide
precursor downstream of the reaction zone of the pyrolysis reactor
to form coated ultrafine titanium dioxide particles with a smooth,
homogeneous metal oxide coating; and (c) isolating the coated
ultrafine titanium dioxide particles.
In one embodiment, the reactor is a pyrolysis reactor. In another
embodiment, the reactor is a flame spray pyrolysis reactor.
In one embodiment, the TiO.sub.2 precursor is TiCl.sub.4 or
titanium tetraisopropoxide. In one embodiment of the invention, the
coated ultrafine titanium dioxide particles are at least 95% by
weight in the rutile form. In another embodiment, the ultrafine
titanium dioxide particles comprise aluminum oxide as a dopant.
In one embodiment, the particle size of the titanium dioxide
particles is between about 15 nm to about 100 nm. In another
embodiment, the particle size is between about 15 nm to about 60
nm. In another embodiment the specific surface area of the
particles is between about 15 m.sup.2/g to about 100 m.sup.2/g.
In one aspect of the invention, the metal oxide coating of the
invention may comprise a metal oxide selected from the group
consisting of SiO.sub.2, Al.sub.2O.sub.3, B.sub.2O.sub.3,
ZrO.sub.2, GeO.sub.2, WO.sub.3, Nb.sub.2O.sub.5, MgO, ZnO and
SnO.sub.2.
In a particular embodiment, the metal oxide coating comprises
SiO.sub.2. In another particular embodiment of the invention, the
metal oxide coating comprises Al.sub.2O.sub.3.
The metal oxide precursor may be introduced using variety of
methods. In one embodiment, the metal oxide precursor is introduced
as a vapor. In another embodiment, the metal oxide precursor is
introduced as an aerosol. In still another embodiment of the
invention, the metal oxide precursor is introduced as a spray,
which may contain one or more solvents.
The metal oxide precursor is introduced into the process when the
majority of the titanium dioxide particles have been formed. In one
embodiment of the invention, the ultrafine TiO.sub.2 particles are
contacted with the metal oxide precursor at a point downstream of
the reaction zone where at least 90% of the TiO.sub.2 precursor has
reacted to form ultrafine TiO.sub.2 particles. In another
embodiment, the ultrafine TiO.sub.2 particles are contacted with
the metal oxide precursor at a point downstream of the reaction
zone where at least 95% of the TiO.sub.2 precursor has reacted to
form ultrafine TiO.sub.2 particles.
The metal oxide precursor may be any compound that produces the
desired metal oxide upon contacting the titanium dioxide particles.
In one embodiment, the metal oxide precursor is selected from
silicon halides, hexaalkyldisiloxanes, tetraalkylorthosilicates and
silicon containing salts. In another embodiment, the metal oxide
precursor is hexamethyldisiloxane or silicon tetrachloride.
In one embodiment, the metal oxide coating is between about 1 nm
and about 10 nm thick. In yet another embodiment, the metal oxide
coating is between about 2 nm and about 4 nm thick.
Also provided herein are ultrafine titanium dioxide particles
comprising a smooth, homogeneous coating of one or more metal
oxides on the surface are provided, wherein the coating is between
about 1 nm and about 10 nm thick or about 2 nm to about 4 nm
thick.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1. Experimental set-up with a flame spray pyrolysis (FSP)
reactor, lower (RBH) and upper glass tubes and the ring for
addition of hexamethyldisiloxane (HMDSO)/N.sub.2.
FIG. 2. Transmission electron microscopy (TEM) images of
4Al/TiO.sub.2 coated with 5 wt. % (a), 10 wt. % (b) and 20 wt. %
(c) SiO.sub.2 by addition of HMDSO vapor at 20 cm RBH and 15 L/min
N.sub.2 through the ring. Segregation in amorphous and crystalline
domains is obtained by premixing Si/Al/Ti in the FSP precursor
solution (d).
FIG. 3. Theoretical coating thickness as a function of silica
content and 4Al/TiO.sub.2 core particle diameter.
FIG. 4. High angle annular dark field detector (HAADF)-STEM image
and energy-dispersive X-ray (EDX) spot analyses of 4Al/TiO.sub.2
coated with 20 wt % SiO.sub.2.
FIG. 5. Rutile weight fraction (left axis, triangles), specific
surface area (SSA) (right axis, circles) as a function of SiO.sub.2
content in Si/4Al/TiO.sub.2 produced from premixed precursor
solutions (filled symbols) and by vapor-doping (open symbols).
FIG. 6. Acetone concentration evolved during the photooxidation of
isopropanol as a function of SiO.sub.2 (circles, lower abscissa)
and Al.sub.2O.sub.3 (triangles, upper abscissa) content.
FIG. 7. Acetone concentration formed during the photooxidation of
isopropanol as a function of RBH.
DETAILED DESCRIPTION OF THE INVENTION
Provided herein are ultrafine titanium dioxide particles coated
with a homogeneous layer of a metal oxide and a gas-phase pyrolysis
process for the preparation of these coated titanium dioxide
particles. The coated titanium dioxide particles of the invention
have a high rutile content and are useful in a variety of
applications, including in polymer composite compositions.
Definitions
The term "ultrafine titanium dioxide" refers to particles of
titanium dioxide that have an average particle size of 1 nm to 100
nm.
The term "primary titania particles" or "primary particles" refers
to the titania particles formed in the reaction zone of the process
before a second coating component has been introduced. The terms
refer to individual particles rather than agglomerates of
particles.
The terms "specific surface area" or "SSA" refer to the surface
area per mass of a material. The units of specific surface area
used herein are m.sup.2/g, or square meters per gram. The terms
"metal oxide precursor" or "coating precursor" refer to a compound
that produces a metal oxide upon contact with oxygen or water
vapor.
The term "reaction zone" is used to refer to the point or position
in the process where TiCl.sub.4 or any other TiO.sub.2 precursor is
reacted with oxygen to form TiO.sub.2.
The term "coating zone" is used to refer to the point or position
in the process where the metal oxide precursor comes in contact
with the pre-formed TiO.sub.2 particles and results in the
formation of a metal oxide coating on the TiO.sub.2 particles.
The term "doped" refers to TiO.sub.2 particles that comprise other
metal oxides in the primary particle. For example, the term
"aluminum-doped" refers to TiO.sub.2 particles that comprise
aluminum oxide in the particles.
The term "halo" or "halogen", as used herein, includes chloro,
bromo, iodo, and fluoro.
The term "silyl halide" refers to a mono-, di-, tri- or tetra-halo
silicon species, for example SiCl.sub.4.
The term "silane" refers to a tetravalent silicon compound, for
example SiH.sub.4 or Si(CH.sub.3).sub.4.
The term "alkyl" is intended to have its customary meaning, and
includes straight, branched, or cyclic, primary, secondary, or
tertiary hydrocarbon, including but not limited to groups with
C.sub.1 to C.sub.10.
The term "aryl" is intended to have its customary meaning, and
includes any stable monocyclic, bicyclic, or tricyclic carbon
ring(s) comprising up to 8 members in each ring (typically 5 or 6),
wherein at least one ring is aromatic as defined by the Huckel 4n+2
rule, and includes phenyl, biphenyl, or naphthyl.
The term "alkoxy" refers to any moiety of the form --OR, where R is
an alkyl group, as defined above.
The term "carrier gas" refers to a gas that carries with it a
concentration of the metal oxide precursor in the vapor phase.
The term "homogeneous coating" as used herein refers to metal oxide
coating that comprises greater than about 75% of one metal oxide,
preferably greater than about 85% of one metal oxide or more
preferably greater than about 95% of one metal oxide.
The term "smooth" referring to a coating of a metal oxide on
titania particles as used herein, means a uniform coating of a
metal oxide on the surface of titania particles that does not
contain segregated areas of amorphous and crystalline content of
the metal oxide and does not contain areas of the particle surface
that do not have a discernable metal oxide coating using the
analytical techniques described herein.
Particles size measurements or ranges herein refer to an average
particle size of a representative sample.
Pyrolysis processes for the production of titania are known and
have been described in detail elsewhere (Madler et al., 2003;
Schulz et al., 2005). One particular pyrolysis process is the flame
spray pyrolysis (FSP) process. The advantages of the flame spray
pyrolysis process for the production of titania include, the
ability to dissolve the precursor directly in the fuel, the
simplicity of introduction of the precursor into the hot reaction
zone (the flame), and flexibility in using the high-velocity spray
jet for rapid quenching of aerosol formation. Flame spray pyrolysis
processes are commonly used to produce ultrafine particles of metal
oxides, and titania in particular.
The surface of titania particles can be passivated by depositing a
coating of another metal oxide to lower the photoactivity of the
titania particles and prevent the photocatalytic decomposition of
substances that incorporate the titania particles. The process
described herein provides a smooth and homogeneous coating layer of
one or more metal oxide on the titanium dioxide nanoparticles
produced by a gas phase oxidation process. In one embodiment of the
invention, the process is a pyrolysis process. In another
embodiment, the process is a flame spray pyrolysis process (FSP).
The coating layer is produced by generation of a vapor phase
containing a metal oxide precursor downstream of the reaction zone
of the reactor. The metal oxide precursor is deposited on the
pre-formed titania nanoparticles as they exit the oxidation
reaction zone and pass through the coating zone of the reactor. The
metal oxide coating is not limited and may comprise any desired
metal oxide depending on the desired characteristics of the coated
nanoparticles. The thickness of the metal oxide coatings produced
by the present invention may be varied by the concentration of the
metal oxide precursor in the coating zone of the reactor. In
addition, the oxide coating can comprise more than one layer of
metal oxide. For example, the process described herein can produce
ultrafine titania particles with separate layers of two or more
metal oxides or mixed metal oxides.
The surface coating of pigmentary titania by the deposition of a
silica precursor after the titania particles have formed has been
described. U.S. Pat. No. 5,562,764 ('764 patent) to Gonzalez
describes a process where SiCl.sub.4 is added downstream of the
TiCl.sub.4 and oxygen reaction zone in a plug flow reactor. The
patent discloses that the SiCl.sub.4 must be added at a temperature
of between about 1200.degree. to about 1600.degree. C. and at a
pressure of between 5-100 psig. Only silicon halides are used as a
silica precursor. International Application Publication No. WO
96/36441 ('441 publication) describes a processes to form
pigment-grade titania coated with a second metal oxide. Similar to
the '764 patent, the '441 publication discloses that the
temperature at which a silica precursor is added to the process
must be greater than 1300.degree. C. to ensure that the silica
precursor completely reacts to form SiO.sub.2. The titania
particles produced by these processes are not ultrafine particles.
The minimum temperature disclosed in the '764 patent and the '441
publication for the addition of the silica precursor is consistent
with the description in the '441 publication that a sufficiently
high temperature is required to enable the silica precursor to form
SiO.sub.2 on the surface of the TiO.sub.2 particles.
U.S. Pat. No. 6,852,306 ('306 patent) to Subramanian et al. and
U.S. Pat. No. 7,029,648 ('648 patent), which is a continuation of
the '306 patent, describes a process for preparation of rutile
titanium dioxide pigment which is coated with a second metal oxide
layer. The '306 and '648 patents disclose that the metal oxide
coating consists of a layer of amorphous oxides of aluminum and
silicon that is not more than 4 nm thick. The composition of the
coating layer is about 1% by weight Al.sub.2O.sub.3 and 1.2% by
weight SiO.sub.2, although SiCl.sub.4 is introduced after the
TiO.sub.2 particles have formed to at least 97%. The '306 and '648
patents disclose that the temperature should be no greater than
about 1200.degree. C. when the SiCl.sub.4 is introduced. A method
to calculate the point of SiCl.sub.4 addition is also
disclosed.
In contrast to the processes described above, the present invention
utilizes a gas phase oxidation process to prepare ultrafine titania
with an average particle size of about 15 nm to about 100 nm. In
one embodiment, the process is a pyrolysis process. In another
embodiment, the process is a flame spray pyrolysis process. The
coating of ultrafine particles presents significant challenges
compared to the coating of larger pigment-grade particles because
the coating of nanoparticles with a smooth, homogeneous coating of
a metal oxide becomes increasingly more difficult as the particle
size of the product decreases. To obtain the same thickness of
coating, ultrafine particles require a larger amount of the
precursor material and this results in a larger amount of the
coating material being present in the product. For instance, for 20
nm particles, it takes about 25% silica to obtain a coating
thickness of 2 nm while for 100 nm particles, it takes only 10%
silica to obtain the same thickness. The use of larger
concentration of coating precursors leads to discrete particles of
silica on inhomogeneous and rough coatings. Furthermore, it is
surprising that ultrafine titania particles produced flame spray
pyrolysis process can be homogeneously coated downstream of the
reaction zone where the temperature is significantly lower. The
processes disclosed in U.S. Pat. No. 5,562,764 and WO 96/36441
indicate that the metal oxide precursor should be added when the
temperature is at least 1200.degree. C. or 1300.degree. C.,
respectively to ensure that the precursor is completely oxidized to
SiO.sub.2. The process described in U.S. Pat. Nos. 6,852,306 and
7,029,648 disclose a lower temperature of addition of SiCl.sub.4,
but describe that the resulting coating layer is an approximately
1:1 mixture of Al.sub.2O.sub.3 and SiO.sub.2.
Although not being bound by theory, the metal oxide coating layer
may be formed by condensation of the metal oxide precursor on the
titania particles followed by oxidation of the precursor to form
the metal oxide. Alternatively, the metal oxide precursor may form
the oxide by a gas-phase oxidation of the precursor followed by
deposition and sintering on the titania particles. These coating
routes were also suggested for gas-phase SiO.sub.2 coating of
TiO.sub.2 in an aerosol flow hot-wall reactor (Powell et al.,
1997). The coating of the titania particles by the process
described herein forms a smooth and homogeneous layer covering the
titania particles.
The titanium dioxide particles produced in pyrolysis reactors by
methods known in the art can have a high rutile form content and a
desired particle size range and morphology depending on the process
parameters and doping of the titanium precursor feed with the
appropriate compound. The production of ultrafine titania by the
pyrolysis process may occur at temperatures from about 600.degree.
C. to about 2400.degree. C. In other embodiments, the titania is
formed at a temperature of between about 600.degree. C. to about
2000.degree. C., between about 600.degree. C. to about 1500.degree.
C. or between 600.degree. C. to about 1000.degree. C.
Titanium dioxide precursors are titanium-containing compounds that
form titanium dioxide when subjected to high temperatures in the
presence of oxygen. Although the process of the invention is not
limited by choice of a particular titanium dioxide precursor,
suitable titanium compounds useful in the invention include, but
are not limited to, titanium alkoxides and titanium halides.
Preferred titanium alkoxides are titanium tetraisopropoxide,
titanium tetraethoxide and titanium tetrabutoxide. Titanium halides
include titanium trichloride and titanium tetrachloride. In a
particular embodiment of the invention, TiCl.sub.4 is used as a
TiO.sub.2 precursor.
Doping certain metal oxide precursors in the feed of the flame
oxidation reaction can impact the form of the titania nanoparticles
produced. Various dopants are added into the flame to control the
characteristics of the powders produced such as the phase
composition, morphology, degree of aggregation, and the primary
particle size. For example, increasing concentration of SiCl.sub.4
in the gas phase production of titania is known to affect the form
of the titania produced by inhibiting the phase transformation of
the anatase form to the rutile form. However, inclusion of aluminum
precursors into the feed of the gas phase oxidation of titania
favors the formation of the rutile form of the product. In flame
oxidation of TiCl.sub.4, addition of 1% to 10% of a volatile
silicon compound has been shown to produce more than 90% anatase
titania while addition of 1% to 10% of an aluminum compound results
in predominantly the rutile form of titania (Vermury and Pratsinis,
1995). The present invention allows the coating of titania
particles with metal oxides without exerting an undesired effect on
the titania primary particles because the titania particles are
pre-formed and are not substantially modified during the coating
step.
The metal oxide coating precursor is optimally introduced at a
point downstream of the reaction zone of the TiO.sub.2 precursor
and oxygen so that the majority of the titania particles are formed
before contact with the coating precursor. In this way,
introduction of the coating precursor will not substantially change
the characteristics of the titania particle. For example,
introduction of a silica precursor after the majority of the
titania particles have substantially completely formed will avoid
the effect of silicon to inhibit the phase transformation from the
anatase form to rutile.
Therefore, it is a significant advantage of the present process to
provide the coating layer of one or more metal oxides after the
titania nanoparticles with the desired characteristics have been
formed in the pyrolysis reactor. For example, the titania particles
formed in the pyrolysis reactor can be formed in the desired rutile
form, which is fixed at this point in the process and not affected
by the subsequent coating with SiO.sub.2. Another significant
advantage of the present invention is that titania with a smooth,
homogeneous coating of a metal oxide can be obtained with a minimum
amount of metal oxide precursor because unnecessary material is not
wasted by unspecific formation of the metal oxide on the titania
particles or agglomeration of the metal oxide material on the
titania particles. Furthermore, with the present process, the metal
oxide coating component is located only on the surface of the
titania particles rather than dispersed throughout the
particles.
In one embodiment of the invention, at least about 70% of the
TiO.sub.2 precursor has been reacted to form titanium dioxide
particles before the metal oxide precursor is introduced into the
product stream. In another embodiment, at least about 80% of the
TiO.sub.2 precursor has reacted to form titanium dioxide particles.
In still another embodiment, at least about 90% of the TiO.sub.2
precursor has reacted to form titanium dioxide particles. In yet
other embodiments, at least 95%, 98%, 99% or 99.5% of the TiO.sub.2
precursor has reacted to form titanium dioxide particles before the
metal oxide is introduced.
The temperature at which the metal oxide coating precursor is
introduced is also an important parameter that impacts the extent
to which the titanium dioxide particles have been completely
formed. In one embodiment, the metal oxide coating precursor is
added at a point in the process where the temperature is less than
about 1300.degree. C. In other embodiments, the coating precursor
is introduced at a temperature of less than about 1200.degree. C.
or less than about 1100.degree. C. In yet further embodiments of
the invention, the coating precursor is introduced into the product
stream at a temperature of less than about 1000.degree. C. or less
than about 900.degree. C.
The titania particles formed in the reactor may be formed with a
pure titanium dioxide precursor, such as TiCl.sub.4, or may include
one or more dopants known in the art to produce titania with
desired characteristics. Dopants include but are not limited to
precursors that produce aluminum oxide, silicon oxide, iron oxide,
zirconium oxide, boron oxide, zinc oxide and tin oxide species in
the titania particles. A combination of dopants may be added to the
process to produce titania particles with desired characteristics.
The precursors may be any compound that may be introduced into the
flame with the TiO.sub.2 precursor, including but not limited to
silanes, silicon halides, alkylhalosilanes or alkylarylsilanes,
silicon alkoxides including tetramethylorthosilicate or
tetraethylorthosilicate and the like; aluminum halides, aluminum
trialkoxides such as aluminum triisopropoxide, aluminum
acetylacetonate and the like. Other precursors include FeCl.sub.3,
ZrCl.sub.4, POCl.sub.3, BCl.sub.3, and Al.sub.2Cl.sub.6. The amount
of the doping component is dependent on the characteristics desired
in the titania particle and the effect of the dopant on the titania
particles. In one embodiment, sufficient dopant is introduced with
the TiCl.sub.4 into the reaction zone of the process to produce
from about 1% to about 50% dopant by weight of the titania
particle. In other embodiments, sufficient dopant is added to
produce from about 0.1% to 10%, about 0.5% to about 5% or from
about 0.5% to about 3% dopant by weight in the titania particle. In
yet another embodiment, the dopant is introduced in a quantity to
provide a concentration of between about 0.5% to about 2% by weight
of the titania particle.
In one embodiment, titanium dioxide nanoparticles doped with an
aluminum oxide precursor are produced in a gas phase oxidation
process to produce a high concentration of the rutile form of the
titanium dioxide with the desired morphology. Aluminum precursors
for use in processes are known in the art. Non-limiting examples of
aluminum precursors include aluminum halides such as AlCl.sub.3,
AlBr.sub.3, AlI.sub.3, AlF.sub.3, Al.sub.2Cl.sub.6, aluminum
trialkoxides, such as Al(OR).sub.3, where R is alkyl or aryl
including aluminum triisopropoxide; and acyl aluminum species such
as aluminum acetylacetonate. It has been reported that doping
TiCl.sub.4 with aluminum produced pure rutile titania while
preparation of titania in the absence of aluminum resulted in
titania mostly in the anatase form (Akhtar and Pratsinis, 1994).
The aluminum precursor may be introduced into the process in
sufficient quantity to produce titania with an Al.sub.2O.sub.3
concentration such that the titania produced has high content of
the rutile form.
The Al-doped titania particles of the present invention comprise
between about 0.1 to about 20% Al.sub.2O.sub.3 by weight of the
titania particle. In other embodiments, sufficient dopant is added
to produce from about 0.1% to 10%, about 0.5% to about 5% or from
about 0.5% to about 3% Al.sub.2O.sub.3 by weight in the titania
particle. In yet another embodiment, the dopant is introduced in a
quantity to provide a concentration of between about 0.5% to about
2% Al.sub.2O.sub.3 by weight of the titania particle. In even
further embodiments, titania particles with 2%, 4%, 6%, 8% or 10%
Al.sub.2O.sub.3 by weight of the titania particles are
produced.
In addition to metal oxide dopants, water vapor or hydrated metal
oxides may be used in the titania reaction. The reaction mixture
may also contain a vaporized alkali metal salt to act as a
nucleant. The alkali metal salts include inorganic potassium salts
including KCl, and organic potassium salts. Cesium salts including
CsCl may also be used in the reaction.
The titania produced by the invention has a high rutile form
content which is desirable in certain optical applications. The
anatase form has a higher photocatalytic activity and can introduce
degradation in polymer matrices in which it is incorporated. In one
aspect of the invention, the titania is at least 50% by weight in
the rutile form. In another embodiment, the titania produced is at
least 60% in the rutile form. In still other embodiments, the
titania produced is at least 70%, 80%, 90% or 95% by weight in the
rutile form. In yet other embodiments, the titania produced is at
least 98% or 99% by weight in the rutile form. Furthermore, the
present invention allows for the coating of the titania particles
with smooth homogeneous metal oxide coatings of metal oxides which
normally promote the formation of the anatase form of titania. For
example, it is known that introduction of silica precursors to the
feed of the titania forming reaction promotes the formation of the
anatase form of titania. The process described herein avoids the
unwanted side effect of doping with silica precursors and provides
titania particles with a high rutile content which are surface
coated with a smooth and homogeneous layer of silica to achieve
enhanced stability of the particles and polymer matrices which
incorporate these materials.
The titania particles produced by the present invention are
ultrafine particles with a particle size of about 15 nm to about
100 nm. As noted previously, there are many uses and advantages to
ultrafine titania compared to pigmentary titania, which has a
larger particle size. Pyrolysis processes are capable of producing
mixed metal oxide particles in the 1-200 nm range from low cost
precursors with production rates up to 250 g/h (Madler, 2002).
Temperature zones and particle residence times in these processes
are key in determining the particle growth. In one embodiment, the
particle size range of the coated titanium dioxide particles is
between about 15 nm to about 80 nm. In another embodiment, the
particle size of the titanium dioxide particles is between about 15
nm to about 60 nm. In still other embodiments, the particle size
range of the particles is between about 15 nm to about 50 nm, about
15 nm to about 40 nm, about 15 nm to about 30 nm or about 15 nm to
about 20 nm.
The specific surface area (SSA) of the titania ultrafine particles
of the present invention may be between about 15 m.sup.2/g and
about 400 m.sup.2. In one embodiment of the invention, the SSA of
the titania particles is between about 15 m.sup.2/g and about 300
m.sup.2/g. In still other embodiments, the SSA of the particles
produced is between about 15 m.sup.2/g and about 200 m.sup.2/g,
about 15 m.sup.2/g to about 100 m.sup.2/g or between about 15
m.sup.2/g to about 70 m.sup.2/g.
The metal oxide coating is not limited to any one specific metal
oxide and may comprise any desired metal oxide depending on the
desired characteristics of the coated nanoparticles. For example,
the nanoparticles of the present invention may be coated with a
smooth, homogeneous layer of one or more of SiO.sub.2,
Al.sub.2O.sub.3, B.sub.2O.sub.3, ZrO.sub.2, GeO.sub.2, WO.sub.3,
Nb.sub.2O.sub.5, MgO, ZnO or SnO.sub.2 by choosing a suitable metal
oxide precursor. The metal oxide coating can also comprise one or
more metal oxide or mixed metal oxides, for example species
described by the formula [SiO.sub.2].sub.x [Al.sub.2O.sub.3].sub.y,
where x=0 to 1 and y=0 to 1. And sum of x and y=1
Suitable metal oxide precursors are any substance that produce the
desired metal oxide upon contact with the nanoparticles. For
example, the nanoparticles may be coated by a homogeneous layer
comprising SiO.sub.2 by introducing a SiO.sub.2 precursor into the
coating zone of the reactor. Silica precursors include any silicon
compound that is a liquid or a gas at the temperature and pressure
of the coating step of the process. The SiO.sub.2 precursors
include but are not limited to silanes, silicon tetrahalides, such
as SiCl.sub.4, SiBr.sub.4, SiF.sub.4 or SiI.sub.4; alkyl or aryl
silylhalides, such as trimethylsilylchloride ((CH.sub.3).sub.3SiCl)
or triphenylsilylchloride; alkyl or aryl silyl di-halides or
tri-halides; hexalkyldisiloxanes, including hexamethyldisiloxane,
(CH.sub.3).sub.3SiOSi(CH.sub.3).sub.3); mono-, di-, tri- or
tetraalkoxysilanes, including tetraalkyl orthosilicates such as
tetraethylorthosilicate or tetramethylorthosilicate and the like,
or tetraaryl orthosilicates; alkylthiosilanes or arylthiosilanes;
tetraalkylsilanes including tetramethyl or tetraethylsilane;
tetraallylsilane; tetraarylsilanes; tetravinylsilanes;
tetrabenzylsilanes; tetralkyl- or tetraaryldisilanes; tetraalkyl-
or tetraaryldisilazanes; trialkyl- or triarylsilylacetates or
sulfonates; and mixtures thereof. It is understood that the silicon
precursor species with a mixture of groups on the silicon are also
used in the invention. For example a compound such as
phenyldimethylchlorosilane is a suitable silica precursor.
Similarly, suitable aluminum oxide, magnesium oxide, zinc oxide and
tin oxide precursors may be used to coat the TiO.sub.2 particles
with the desired oxide. For example suitable aluminum oxide
precursors include but are not limited to aluminum halides
including AlX.sub.3 and Al.sub.2X.sub.6, where X is chloro, bromo,
iodo or fluoro; aluminum trialkoxides (Al(OR).sub.3 including
aluminum triisopropoxide; aluminum acyl compounds including
aluminum acetylacetonate; and tetralkyldialuminoxanes
(R.sub.2Al--O--AlR.sub.2).
Certain metal oxide precursors will be volatile at the temperatures
used in the process. For example, SiCl.sub.4 has a boiling point of
57.degree. C. and hexamethyldisiloxane has a boiling point of
101.degree. C. and can be vaporized easily with a carrier gas. In
some cases, the metal oxide precursors will need to be pre-heated
to produce the desired concentration in the gas phase for coating
the titania particles. For example, it is known in the art that
AlCl.sub.3 must be heated to achieve a sufficient vapor pressure of
AlCl.sub.3 to transport controlled amounts of the compound by a
carrier gas. In certain embodiments, the metal oxide precursor may
be cooled to lower the vapor pressure of the material and lower the
concentration in the feed to the process.
The metal oxide coating precursor is introduced into the process
downstream of the flame reaction zone by any suitable methods known
in the art. As described above, it is most beneficial to introduce
the metal oxide precursor at a point downstream of the TiO.sub.2
particle formation so that the characteristics of the titania
particles are not altered by the coating of the particles. The
selection of the metal oxide precursor injection point is chosen
based on the extent of formation of the TiO.sub.2 particles. The
metal oxide coating precursor may be introduced into the process as
a vapor carried by a gas or in the form of an aerosol through one
or more slots or openings downstream of the flame reaction zone.
The metal oxide precursor may be pre-heated prior to introduction
into the reactor. Sprays of a liquid metal oxide precursor,
optionally in a suitable solvent, may be used. In other
embodiments, the coating precursor may be introduced into the
coating zone through a porous wall element or through openings in a
tube element placed downstream of the flame reaction zone. It will
be apparent to persons skilled in the art that the number of
openings or nozzles used to introduce the metal oxide precursor is
not limited and may be adjusted to produce the desired
concentration of the metal oxide precursor in the coating zone of
the reactor.
The preparation of aerosols is well known in the art and this
technology is used to prepare aerosols of non-volatile metal oxide
precursors. For example, an aerosol of an oxide precursor may be
formed from an aqueous solution of the metal oxide precursor with
an ultrasonic nebulizer or other suitable means. Salt solutions of
the oxide precursors may also be used to form aerosols.
In one embodiment of the invention, a metal oxide coating may be
applied to the pre-formed TiO.sub.2 particles downstream of the
reaction zone by introducing a metal oxide coating precursor with
oxygen into a second flame in a pyrolysis reactor to produce the
metal oxide. The metal oxide formed in this matter is introduced
into the reactor so that it comes into contact with the pre-formed
TiO.sub.2 particles. Methods known in the art may be used to
produce the metal oxide in this manner, and any of the metal oxide
precursors discussed herein may be used with this embodiment of the
invention. It will be apparent to one skilled in the art that the
parameters of the second flame, including flow rate of the metal
oxide precursor and oxygen as well as the temperature of the flame
can be controlled to optimum settings depending on the specific
metal oxide required for the coating layer.
Once the metal oxide precursor has contacted the titania particles
and deposited a coating of the metal oxide, the coated titania
particles are isolated or collected downstream of the coating zone.
Any suitable methods known in the art may be used to isolate or
collect the coated titania particles. For example, the coated
particles may be isolated on a filter.
The thickness of the metal oxide coating is controlled by the
concentration of the metal oxide precursor in the coating zone of
the pyrolysis reactor. In general, higher concentrations of metal
oxide precursor will result in thicker coating layers on the
ultrafine titania particles. When the metal oxide precursor is a
liquid, the vapor can be generated by passing a carrier gas through
a solution of the liquid. For example, a suitable carrier gas may
be bubbled through a liquid sample of SiCl.sub.4 or HMDSO to
generate a vapor phase with a desired concentration of the
SiO.sub.2 precursor. The concentration of the metal oxide precursor
vapor in the coating zone can be adjusted by the flow rate of the
carrier gas and the temperature of the precursor liquid. Higher
concentrations of the metal oxide precursor are achieved by
increasing the rate that the carrier gas passes through the liquid
or by increasing the temperature of the liquid, which increases the
vapor pressure and the concentration of the metal oxide precursor
in the vapor phase. These parameters are easily adjusted by
standard methods known in the art.
The concentration of the metal oxide precursor in the vapor phase
may be adjusted to obtain titania particles with a concentration of
between about 1% to about 50% by weight of the metal oxide coating
component in the coated product. In one embodiment, the
concentration of the metal oxide coating component in the coated
titania particles is between about 1% to about 30% by weight. In
other embodiments, the concentration of the metal oxide coating
component in the titania product is between about 1% to about 20%,
about 2% to about 15%, about 2.5% to 10%, about 5% to about 20%, or
about 10% to about 20% by weight of the coated product.
The coated ultrafine titania particles of the present invention
will contain metal oxide coating layers about 1 nm to about 10 nm
thick. In one embodiment, the metal oxide coating layer is from
about 2 nm to about 8 nm thick. In other embodiments, the metal
oxide coating layer is from about 2 nm to about 6 nm thick or from
about 2 nm to about 4 nm thick.
In one exemplary embodiment, titania particles prepared by a
pyrolysis process are coated with a smooth, homogeneous SiO.sub.2
coating by introducing a silica precursor downstream of the flame
reaction zone where TiCl.sub.4 and oxygen form TiO.sub.2 particles.
Hexamethyldisiloxane (HMDSO) is introduced as a vapor into the
pyrolysis reactor downstream of the TiO.sub.2 particle formation
zone by a carrier gas. The HMDSO contacts the titania ultrafine
particles and is oxidized to form a smooth and homogeneous coating
of SiO.sub.2 on the surface of the titania particles. It is
understood that the process is not limited to HMDSO but any silica
precursor may be used that is a gas or liquid at the temperature
and pressure of the coating step. For example, SiCl.sub.4 or
Si(CH.sub.3).sub.4 or other suitable precursors may be used in
place of HMDSO. The efficiency of the titania surface passivation
is measured by the ability of the coated particles to catalyze the
photo-induced conversion of isopropanol to acetone. Particles that
have been more effectively passivated by the deposition of a smooth
and homogeneous coating of SiO.sub.2 on the surface will have a
lower catalytic activity, demonstrated by a lower amount of acetone
produced.
FIG. 1 shows a schematic of an example flame spray pyrolysis
reactor used to produce the ultrafine titanium dioxide particles of
the present invention comprising a homogeneous coating of a metal
oxide. The embodiment illustrated is not intended to be limiting
but to provide an example of the process for coating one possible
metal oxide on titania nanoparticles. Solutions of a titanium
dioxide precursor and an aluminum oxide precursor are fed through
an inner nozzle capillary and dispersed by oxygen gas flow supplied
through a surrounding annulus. The concentration of the solutions
is not critical but can be adjusted to produce the desired
concentration of TiO.sub.2 precursor and aluminum dopant in the
flame oxidation zone. In the illustrated embodiment, the precursor
solution spray is ignited by a ring-shaped, methane/oxygen premixed
flame at the nozzle base to produce nanoparticles of 2, 4, 6, 8 or
10 wt. % Al.sub.2O.sub.3 in TiO.sub.2.
For comparison, nanoparticles containing silicon dioxide were also
prepared by adding a silica precursor to the Al/TiO.sub.2 precursor
solution prior to introduction into the flame. This is analogous to
processes where the TiCl.sub.4 and silica precursors are introduced
into the reaction zone of the process simultaneously. The materials
produced are designated as xSi/TiO.sub.2, yAl/TiO.sub.2 or
xSi/yAl/TiO.sub.2 depending on the weight fraction of
Al.sub.2O.sub.3 and SiO.sub.2 present in the TiO.sub.2. For
example, 10Si/TiO.sub.2 contains 10 wt % SiO.sub.2 and 90 wt. %
TiO.sub.2, while 10Si/.sub.4Al/TiO.sub.2 is composed of 10 wt. %
SiO.sub.2, 4 wt. % Al.sub.2O.sub.3 and 86 wt. % TiO.sub.2.
FIG. 1 shows a quartz glass tube of varying length, designated
herein as ring burner height (RBH), that surrounds the flame
oxidation zone. In one embodiment, the inner diameter of the quartz
tube was 4.5 cm, and the RBH was varied from 5, 10, and 20 to 30
cm.
At the upper edge of the glass tube, surrounding the spray aerosol,
is placed a metal torus ring which contains varying equidistant
radial openings. Through the openings in the torus ring, a metal
oxide precursor carried by a gas is injected into the path of the
aerosol spray, thereby depositing a homogeneous layer of metal
oxide on the pre-formed Al-doped TiO.sub.2 nanoparticles. The
carrier gas is not important as long as it does not react with the
metal oxide precursors or with the titania particles. Suitable
carrier gasses include nitrogen, argon and other non-reactive
gasses. In the illustrated embodiment, nitrogen gas carrying HMDSO
is injected along with additional nitrogen to keep constant either
the power input (defined as the ring N.sub.2 kinetic energy) or the
total nitrogen flow rate. For example, HMDSO is added through 16
openings.
In the illustrated embodiment, the diameter of the metal torus ring
is 4.5 cm and the diameter of the metal pipe is 3.8 mm. The number
of openings in the metal torus can be varied depending on the
diameter of the torus ring and the desired coverage area. In the
illustrated embodiment, the metal torus ring contained 1, 2, 4, 8
or 16 openings that were 0.6 mm in diameter. However, it will be
apparent to one skilled in the art that the number of openings or
the diameter of the openings is not limited and can be varied to
achieve the desired concentration of metal oxide precursor in the
coating zone depending on the amount of coating layer desired on
the titania particles.
A second quartz glass tube is placed above the vapor ring. Once the
particles are coated by the metal oxide, the nanoparticles are then
directed to a filter unit that collects the product.
The present invention will be understood more readily by reference
to the following examples, which are provided by way of
illustration and are not intended to be limiting of the present
invention.
EXAMPLES
Experimental
Preparation of Al.sub.2O.sub.3/TiO.sub.2 Particles and In-Situ
Coating with SiO.sub.2
Precursor solutions (1 M) were prepared from
titanium-tetra-isopropoxide (TTIP, Aldrich, purity>97%) and
aluminum sec-butoxide (Al(s-BuO).sub.3), Aldrich, purity>97%) in
xylene (Fluka, >98.5%) resulting in 2, 4, 6, 8 or 10 wt %
Al.sub.2O.sub.3/TiO.sub.2. SiO.sub.2/TiO.sub.2 or
SiO.sub.2/Al.sub.2O.sub.3/TiO.sub.2 particles were produced by
adding hexamethyldisiloxane (HMDSO, Aldrich, purity>99%) to
these precursor solutions. All solutions were fed at 5 ml/min
through the inner nozzle capillary and dispersed by 5 l/min oxygen
(Pan Gas, purity>99%) supplied through the surrounding annulus.
The pressure drop at the nozzle tip was maintained at 1.5 bar. The
precursor solution spray was ignited by a ring-shaped,
methane/oxygen (1.5/3.2 l/min) premixed flame at the nozzle
base.
The experimental set-up for in-situ silica-coating of
aluminum-doped titania particles made by a flame spray pyrolysis
(FSP) process is shown in FIG. 1. The FSP reactor was operated as
described previously producing Al-doped TiO.sub.2. The flame was
sheathed with 40 l/min O.sub.2 and surrounded by a quartz glass
tube (ID=4.5 cm) 5, 10, 20 or 30 cm in length (ring-burner-height:
RBH). At its upper edge, a metal torus (pipe diameter=3.8 mm) ring
(ID=4.5 cm) with 1, 2, 4, 8 or 16 equispaced radial openings, 0.6
mm each in diameter, was placed surrounding the spray aerosol.
Through these openings, N.sub.2 gas carrying HMDSO vapor as coating
precursor was injected along with additional 5, 10, 15, 20, 25 or
30 L/min N.sub.2. The additional N.sub.2 flow rate was chosen to
keep constant either the power input (defined as the ring N.sub.2
kinetic energy) or the total N.sub.2 flow rate. Standard conditions
applied, unless otherwise stated, refer to a spray flame producing
4Al/TiO.sub.2 (23.6 g/h) with HMDSO added through 16 openings at 20
cm RBH and 15 L/min additional N.sub.2 (Table 1). This corresponds
to a power input of 0.4 W. Above the vapor doping ring another
quartz tube 30 cm (ID=4.5 cm) long was placed.
Process parameters for in-situ SiO.sub.2 coating of
Al.sub.2O.sub.3/TiO.sub.2 are shown in Table 1 below. The standard
settings applied if not otherwise stated are shown in bold.
TABLE-US-00001 TABLE 1 Process Parameters wt % Al.sub.2O.sub.3 4,
6, 8, 10 Ring-burner-height 5, 10, 20, 30 RBH, cm Ring N.sub.2,
L/min 5, 10, 15, 20, 25, 30 wt % S1O.sub.2 2.5, 5, 10, 15, 20 Upper
tube length, cm 5, 10, 20, 30
The HMDSO vapor was supplied either from a bubbler or an evaporator
(Bronkhorst). In the bubbler, liquid HMDSO was in a glass flask
immersed in a silicon oil bath maintained at 10.5.degree. C. and a
N.sub.2 stream (0.8 L/min) was bubbled through the HMDSO for 20 wt
% SiO.sub.2 coating (5.9 g/h SiO.sub.2) in the product powder
(standard; Table 1). To achieve lower metal oxide concentrations
(2.5, 5, 10, 15 wt % SiO.sub.2) the temperature of the bubbler was
reduced to 4.5.degree. C. and the nitrogen flow rate adjusted to
yield the desired SiO.sub.2 production rate. The evaporator was
used when the pressure drop over the vapor doping ring became too
high for operation of the bubbler. The evaporator and its exit
manifold were heated to 75.degree. C. as the HMDSO was carried by
0.8 L/min N.sub.2. That HMDSO/N.sub.2 stream was mixed immediately
at the exit of the units with the additional N.sub.2 and fed to the
torus ring above the FSP nozzle. The BET nitrogen adsorption and
X-ray diffraction (XRD) properties were similar when using either
evaporator or bubbler verified for 2 openings at 1.0 W (5 L/min
N.sub.2).
Particle Characterization
The product powders were deposited onto a holey carbon foil
supported on a copper grid for further analysis by transmission
electron microscopy (TEM, microscope: CM30ST, LaB.sub.6 cathode)
and scanning transmission electron microscopy (STEM, microscope:
Tecnai F30, field emission cathode). Both microscopes were operated
at 300 kV and have a SuperTwin lens with a point resolution of
.about.2 .ANG. (Fa. FEI, Eindhoven). STEM images were recorded with
a high angle annular dark field detector (HAADF). The presence of
Ti, Al and Si at selected spots in STEM images was determined by
energy-dispersive X-ray (EDX) analysis (Fa. EDAX detector).
X-ray diffraction (XRD) patterns were obtained with a Bruker AXS D8
Advance diffractometer (40 kV, 40 mA, Karlsruhe, Germany) operating
with Cu K.sub..alpha. radiation. The anatase and rutile crystallite
sizes, x.sub.a and x.sub.r, respectively, and phase composition
were determined by the fundamental parameter approach and the
Rietveld method (Cheary and Coelho, 1998). The BET powder-specific
surface area (SSA), was measured by nitrogen adsorption at 77 K
(Micromeritics TriStar) after degassing the samples, at least, for
1 h at 150.degree. C. in nitrogen.
The photooxidation of isopropyl alcohol using the FSP-made
particles was studied by monitoring the released acetone
concentration by gas chromatography after 30 minutes reaction time.
Inhibition of the photocatalytic activity of titania by the
SiO.sub.2 surface coating was demonstrated by the amount of acetone
produced from isopropyl alcohol. Lower amounts of acetone produced
in the photocatalytic degradation reaction is indicative of a lower
activity of the coated titania particles.
Example 1
Influence of Silica Content on Particle Morphology
FIG. 2 shows Al-doped TiO.sub.2 particles coated with 5 wt. % (a),
10 wt. % (b) and 20 wt. % (C) SiO.sub.2. At 5 wt % SiO.sub.2 (FIG.
2a) coatings are not visible in the TEM image but a thin amorphous
layer could still be present on the particle surface. All Al-doped
titania particles with 10 wt. % (FIG. 2b) and 20 wt % SiO.sub.2
(FIG. 2c) are homogeneously coated with a homogeneous layer of
SiO.sub.2 2-4 nm thick. Separate amorphous particles were not
observed at any SiO.sub.2 content. In contrast, when HMDSO is
introduced with the TiO.sub.2 precursor solution, particles
segregated in amorphous and crystalline domains are obtained (FIG.
2d) along with some coated particles. This is analogous to the
process described in U.S. Pat. No. 7,083,769, which describes the
introduction of a silica precursor (SiCl.sub.4) simultaneously with
the TiCl.sub.4 into the flame of a FSP reactor. The segregation of
amorphous silica and crystalline titania was confirmed by EDX and
STEM analysis, as has also been shown for SiO.sub.2/TiO.sub.2 at
the same composition from vapor flames (Teleki et al., 2005).
Homogeneously SiO.sub.2 coated TiO.sub.2 particles could only be
produced in these flames by a rapid cooling of the flame by nozzle
quenching (Teleki et al., 2005).
The theoretical coating thickness as a function of silica content
and alumina/titania core particle diameter is shown in FIG. 3. For
example, coating 40 nm core particles with 5 wt % SiO.sub.2 results
in a layer thickness of <1 nm (FIG. 3) that is not visible in
TEM (FIG. 2a). Adding 20 wt % to the same core particles results in
a theoretical coating thickness of 2-3 nm (FIG. 3) that is in
agreement with TEM (FIG. 2c) thus closing the mass balance for the
system.
The 20Si/4Al/TiO.sub.2 particle morphology (FIG. 2c) was further
investigated by EDX analysis. In FIG. 4 the HAADF-STEM image and
the corresponding Ti and Si spot analyses are shown. Because of
atomic number (Z) contrast (Ti scatters stronger, i.e. appears
brighter, than Si), the areas with silica at the rim of the
particles appear significantly darker than the core regions
comprising titania. Consequently, spot analysis of the dark region
show only the signal of Si whereas spectra of the core show Ti as
the main peak, a very small Al peak and a Si peak that is caused
from the silica coating above and below the titania crystal.
In FIG. 5 the rutile weight fraction (triangles, left axis) and SSA
(circles, right axis) of Si/4Al/TiO.sub.2 from premixed precursor
solutions (filled symbols) or HMDSO vapor-coated (open symbols) are
shown. For pure 4Al/TiO.sub.2 the rutile content is nearly
identical using the open or enclosed set-up, respectively. Notably,
the rutile weight fraction decreases with increasing silica content
as HMDSO is added to the FSP precursor solution (FIG. 5, filled
triangles) where the silica precursor is introduced into the flame
with TiCl.sub.4. For example, already addition of 1.5 wt %
SiO.sub.2 reduces the rutile content from initial 65 to 54 wt % and
further to 33 wt % at 10 wt % SiO.sub.2. However, the anatase
promotion of silica is counteracted by the presence of alumina
compared to pure SiO.sub.2/TiO.sub.2, For 10Si/TiO.sub.2 only 7 wt
% rutile was obtained (not shown). In contrast, for the process of
the present invention utilizing HMDSO vapor-coated 4Al/TiO.sub.2
where the SiO.sub.2 coating is achieved after the primary
4Al/TiO.sub.2 particles have formed, the rutile content is nearly
constant at 70 wt % up to 15 wt % SiO.sub.2 and decreases to 60 wt
% at 20 wt % SiO.sub.2. Thus the anatase promotion of silica is
significantly reduced by addition of HMDSO vapor after
4Al/TiO.sub.2 particle formation in the flame.
FIG. 6 depicts micrograms (.mu.g) of acetone evolving per
milliliter (ml) of isopropyl alcohol (IPA) as a function of
SiO.sub.2 content in vapor-coated 4Al/TiO.sub.2 particles (circles,
lower abscissa). The amount of acetone formed from isopropyl
alcohol is a measure of the photocatalytic activity of the coated
TiO.sub.2 nanoparticles. Pure 4Al/TiO.sub.2 particles form 225
.mu.g acetone. This remains rather constant at 2.5 wt % SiO.sub.2,
but is then rapidly reduced to 149 and 26 .mu.g acetone by addition
of 5 and 10 wt % SiO.sub.2, respectively. At 2.5 wt % SiO.sub.2 the
coating should be rather thin (<0.5 nm, FIG. 3), and is thus not
sufficient to hinder the photocatalytic activity of TiO.sub.2. The
coating thickness increases with increasing silica content (FIG. 3)
and at 10 wt % coatings visible in TEM have formed (FIG. 2b).
Further increasing the silica content to 15 and 20 wt % has rather
little effect on the acetone formed (FIG. 6). This indicates that
the coating thickness formed at 10 wt % SiO.sub.2 is sufficient to
inhibit the photocatalytic decomposition of IPA with TiO.sub.2.
FIG. 6 also shows how the photocatalytic activity can be further
reduced by increasing the alumina content in the core particles at
constant silica content (triangles, upper abscissa). At 6 wt %
Al.sub.2O.sub.3 only 4 .mu.g acetone is evolving and a complete
inhibition is achieved at 10 wt % Al.sub.2O.sub.3. Increasing the
alumina content increases the rutile content of the particles, e.g.
at 10 wt % Al.sub.2O.sub.3 the rutile content is 83 wt %. The lower
photocatalytic activity of rutile compared to anatase titania has
been reported in the literature (Allen et al., 2005).
Example 2
Effect of Ring-Burner-Height (RBH)
FIG. 7 shows the evolution of acetone as a function of RBH. The
acetone formed decreases from 99 to 7 .mu.g acetone as the RBH is
increased from 5 to 30 cm, respectively. This is a result of the
increasing rutile content as well as the formation of homogeneous
SiO.sub.2 coatings with increasing RBH.
REFERENCES
Akhtar, M. K., S. E. Pratsinis, and S. V. R. Mastrangelo, "Dopants
in vapor-phase synthesis of titania powders," J. Am. Ceram. Soc.
75, 3408 (1992). Akhtar, M. K., S. E. Pratsinis, and S. V. R.
Mastrangelo, "Vapor-phase synthesis of Al-doped titania powders,"
J. Mater. Res. 9, 1241 (1994). Allen, N. S., M. Edge, G. Sandoval,
J. Verran, J. Stratton, and J. Maltby, "Photocatalytic coatings for
environmental applications," Photochem. Photobiol. 81, 279 (2005).
Allen, N. S., H. Khatami, and F. Thompson, "Influence of
titanium-dioxide pigments on the thermal and photochemical
oxidation of low-density polyethylene film," Eur. Polym. J. 28, 817
(1992). Beecroft, L. L., and C. K. Ober, "Nanocomposite materials
for optical applications," Chem. Mat. 9, 1302 (1997). Braun, J. H.,
"Titanium dioxide--A review," J. Coat. Technol. 69, 59 (1997).
Carotenuto, G., Y. S. Her, and E. Matijevic, "Preparation and
characterization of nanocomposite thin films for optical devices,"
Ind. Eng. Chem. Res. 35, 2929 (1996). Caseri, W. R.,
"Nanocomposites of polymers and inorganic particles: preparation,
structure and properties," Mater. Sci. Technol. 22, 807 (2006).
Cheary, R. W., and A. A. Coelho, "Axial divergence in a
conventional X-ray powder diffractometer. I. Theoretical
foundations," J. Appl. Crystallogr. 31, 851 (1998). Christensen, P.
A., A. Dilks, T. A. Egerton, and J. Temperley, "Infrared
spectroscopic evaluation of the photodegradation of paint--Part II:
The effect of UV intensity & wavelength on the degradation of
acrylic films pigmented with titanium dioxide," J. Mater. Sci. 35,
5353 (2000). Egerton, T. A., "The modification of fine powders by
inorganic coatings," KONA 16, 46 (1998). El-Toni, A. M., S. Yin,
and T. Sato, "Control of silica shell thickness and microporosity
of titania-silica core-shell type nanoparticles to depress the
photocatalytic activity of titania," J. Colloid Interface Sci. 300,
123 (2006). Hung, C. H., and J. L. Katz, "Formation of mixed-oxide
powders in flames 1. TiO.sub.2--SiO.sub.2," J. Mater. Res. 7, 1861
(1992). Iler, R. K., Product comprising a skin of dense, hydrated
amourphous silica bound upon a core of another solid material and
process of making same, U.S. Pat. No. 2,885,366 (1959). Kodas, T.
T., Q. H. Powell, and B. Anderson, Coating of TiO.sub.2 pigment by
gas-phase and surface reactions, International patent, WO 96/36441
(1996). Kyprianidou-Leodidou, T., P. Margraf, W. Caseri, U. W.
Suter, and P. Walther, "Polymer sheets with a thin nanocomposite
layer acting as a UV filter," Polym. Adv. Technol. 8, 505 (1997).
Lee, S. K., K. W. Chung, and S. G. Kim, "Preparation of various
composite TiO.sub.2/SiO.sub.2 ultrafine particles by vapor-phase
hydrolysis," Aerosol Sci. Technol. 36, 763 (2002). Madler, L., H.
K. Kammler, R. Mueller, and S. E. Pratsinis, "Controlled synthesis
of nanostructured particles by flame spray pyrolysis," J. Aerosol
Sci. 33, 369 (2002). Madler, L., W. J. Stark, and S. E. Pratsinis,
"Simultaneous deposition of Au nanoparticles during flame synthesis
of TiO.sub.2 and SiO.sub.2," J. Mater. Res. 18, 115 (2003). Mezey,
E. J., "Pigments and Reinforcing Agents" in Vapor Deposition, eds.
C. F. Powell, J. H. oxley and J. M. Blocher Jr., John Wiley &
sons, New York (1966). Nussbaumer, R. J., W. R. Caseri, P. Smith,
and T. Tervoort, "Polymer-TiO.sub.2 nanocomposites: A route towards
visually transparent broadband UV filters and high refractive index
materials," Macromol. Mater. Eng. 288, 44 (2003). Piccolo, L., B.
Calcagno, and E. Bossi, Process for the post-treatment of titanium
dioxide pigments, U.S. Pat. No. 4,050,951 (1977). Powell, Q. H., G.
P. Fotou, T. T. Kodas, B. M. Anderson, and Y. X. Guo, "Gas-phase
coating of TiO.sub.2 with SiO.sub.2 in a continuous flow hot-wall
aerosol reactor," J. Mater. Res. 12, 552 (1997). Schulz, H., L.
Madler, R. Strobel, R. Jossen, S. E. Pratsinis, and T. Johannessen,
"Independent control of metal cluster and ceramic particle
characteristics during one-step synthesis of Pt/TiO.sub.2," J.
Mater. Res. 20, 2568 (2005). Song, K. C., and S. E. Pratsinis,
"Synthesis of bimodally porous titania powders by hydrolysis of
titanium tetraisopropoxide," J. Mater. Res. 15, 2322 (2000).
Teleki, A., S. E. Pratsinis, K. Kalyanasundaram, and P. I. Gouma,
"Sensing of organic vapors by flame-made TiO.sub.2 nanoparticles,"
Sens. Actuators, B, Chem 119, 683 (2006). Teleki, A., S. E.
Pratsinis, K. Wegner, R. Jossen, and F. Krumeich, "Flame-coating of
titania particles with silica," J. Mater. Res. 20, 1336 (2005).
Vemury, S., and S. E. Pratsinis, "Dopants in flame synthesis of
titania," J. Am. Ceram. Soc. 78, 2984 (1995). Wegner, K., and S. E.
Pratsinis, "Nozzle-quenching process for controlled flame synthesis
of titania nanoparticles," AlChE J. 49, 1667 (2003).
* * * * *